Shape memory alloys (SMAs), typically but not limited to Ni—Ti alloy, possess unique characteristics that permit them to generate a large strain output and force. The SMAs are also flexible in nature permitting high pseudoelasticity. It can easily operate as an On/Off type actuator, and it has found some applications in various fields ranging from medical, to industry, and to aerospace. Wider application of SMAs in these and other fields has been proposed, in such fields as space and aerospace, precise mechanical or optical systems, and in robotics, but this wider application has been so far limited due to the difficulty in achieving a precise strain output regulation. This is primarily because the SMA mechanism is intrinsically thermo-activated through so-called Martensite to Austenite phase transformation. The correlation between the strain output of SMA and its temperature, which holds the key for strain output regulation, is found to be highly nonlinear. Hysteresis is a significant problem. It is not known what factors affect the local temperature at a given point in the SMA, and to what extend these factors are relevant, although it has been suggested that phase state, loading stress, and fatigue cycles, may be relevant.
A first known method of strain output regulation is based on a thermomechanical model. For this method, a thermomechanical model of the SMA must first be established, and then given a model strain output regulation can be computed for a given thermal state. For example, see Ikuta, K., M. Tsukamoto, S. Hirose. “Mathematical Model and Experimental Verification of Shape Memory Alloy for Designing Micro Actuator”, Proceedings, IEEE Micro Electro Mechanical Systems, New Jersey, pp. 103-108, 1991, and Prahlad, H., I. Chopra. “Comparative Evaluation of Shape Memory Alloy Constitutive Models with Experimental Data”, Journal of Intelligent Material Systems and Structures: Vol. 12, pp. 383-395, June 2001. The accurate modeling of strain output permits the definition of algorithms for strain output regulation.
The first method suffers several drawbacks: (1) the model parameters typically are determined experimentally (or are not accurate) because they depend on the specific type of SMA actuator (alloy composition and heat-treatment) and on the external thermal environmental parameters; (2) there has not been a widely applicable model developed that permits generalization to different situations; and (3) control schemes based on such thermomechanical models are very complex due to the need to deal with SMA's inherent hysteresis with minor loops. Because of these challenges, the method is more of a theoretical research tool than a practical method for controlling SMA actuators.
A second method is a more typical feedback control of a SMA's strain output using a position sensor. Typically a high accuracy position sensor, such as a linear variable differential transformer (LVDT) or an optical encoder is used to provide strain feedback to a controller. Although it has proven strain output control precision, it requires dedicated position sensors and sophisticated power supply hardware with signal amplification functions in order to provide the feedback. The power supply requirements makes the SMA actuator control system bulky. For example the requirement for the position sensor, which has to be placed somehow at the moving end of SMA actuator, has been the major constraint on the design and the use of SMA in a number of potential applications. Such applications include space and aerospace systems, where there are very stringent requirements for the mass, volume and design simplicity. This is particularly challenging if multiple SMA actuators need to be used, for multiple actuation states, etc.
A third known method is a multi-step strain output control of segmented SMA actuators [8]. Such methods required a compound SMA actuator having of a number of SMA segments and each segment can be turned on or off individually (binary control) so as to achieve a multistep strain output regulation for the integrated actuator. This method also presents drawbacks which include: (1) it permits only stepwise rather than continuous strain output regulation, and the size of the steps is limited by the number of segments; and (2) because the SMA is intrinsically thermo-activated, when one SMA segment is activated through electrical heating, heating will typically spread to neighboring segments, which can cause partial activation of those segments, thus resulting in relatively poor strain output regulation precision. While smaller segments are subject to less hysterisis than larger volumes, they are also more sensitive to temperature changes, such as those induced from neighbouring segments. The addition of insulation between the segments slows down the cooling thermal response of the segments, and also limits the coupling of the mechanical actuation of the segments together resulting in a more failure prone and complicated actuator.
It is noted that this third method defers the problem to statistics rather than addressing it. The control of the segments is exactly the same problem as the whole before, but the number of segments changes the system to a distributed control architecture which has advantages and drawbacks.
An article by Maria Marony Sousa Farias Nascimento et al. of the Department of Electrical Engineering, Universidade Federal de Campina Grade, Brazil, entitled “Electro-thermomechanical characterization of Ti—Ni shape memory thin wires”, teaches measurement of hysteretic strain—temperature and resistance—temperature characteristic curves to determine shape memory parameters, like martensitic transformation temperatures, temperature hysteresis, temperature slopes and shape memory effect under load. While the article does compute a compound hysterisis graph from these two, there is no teaching or suggestion of what parameters are necessary or sufficient to produce precise strain output regulation of a SMA. The hysteretic characterization is one problem that is shown to vary with actuators and setups.
The specific test apparatus disclosed in the article uses a transistor in the design of a voltage/current converter, along with an amplifier and reference resistor to constitute the whole power regulator, which is able to convert a triangle waveform of voltage signal to the exact waveform of current signal to meet the need of SMA characterization experiment. The combined use of amplifier and transistor in that case is believed to compensate the foreseeable current fluctuation upon the voltage signal (because the resistance of SMA changes as it is being heated) so that the current waveform can be exactly the same as the voltage signal waveform. This also explains why it is called the “voltage/current convert”.
WO 2005/075823 to Featherstone et al. teaches a controller for a SMA actuator that includes an electric power source for applying an electric current through an SMA element, a sensor to detect change in an electric resistance of the element; and a regulator for controlling the magnitude of the applied electric current. Unfortunately this teaches a regulator that is bulky, an expensive as noted above. The idea of supply different current when maintaining actuation and when switching actuation is taught.
It is known in the field of SMA heaters that control of a Nitinol heater can be effected using temperature data feedback from a separate temperature sensor such as a thermocouple, or using the resistance within a heater element itself as a temperature sensor, since the resistance of the Nitinol changes with temperature in a predictable way. See U.S. Pat. No. 6,410,886 to Julien. To within the sensitivity requirements of a space heater (not very high), when there is an assumption of no loading stress, the resistance can be used to estimate the temperature so as to estimate the state of SMA heating element. The hysterisis of the heater would be significantly greater than that of the Nitinol elements. No specific temperature and resistance correlation is plotted, and this would be a basic requirement for high precision control feedback. Julien teaches little of practical use for applications of precision control feedback systems. It is inferable from Julien that an ohm meter is used for determining resistance, and thus a device with a dedicated power supply would be integrated into their system.
There is a need for a SMA strain output regulation technique that provides high precision control over the SMA, especially for SMA actuators that encounter variable mechanical resistance during actuation. The need is especially felt for a strain output regulating feedback technique that does not rely on sensors requiring externally driven modulated power sources to operate. Finally, a feedback design for controlling SMA actuators is provided.